The present invention relates to a thin-film transistor and, more particularly, to a thin-film transistor (element) for use in a driving circuit for a liquid-crystal display device and to a method of fabricating the same.
At present, liquid-crystal display devices driven by thin-film transistors (TFTs) as thin-film semiconductor elements are used widely in a notebook personal computer, a vehicle navigator, and the like and requested to be further reduced in size, weight, and cost in the future. To respond to the request, there has been developed a polycrystalline silicon thin-film transistor which allows integral formation of even driving circuits for a pixel portion with a substrate having a display portion and the pixel portion for the display portion formed therein and higher performance thereof has been pursued. Referring to the drawings, a method of fabricating a conventional polycrystalline silicon thin-film transistor will be described.
FIG. 1 is a structural cross section of a thin-film transistor of the type termed xe2x80x9ctop-gatexe2x80x9d produced in accordance with a conventional method. In the drawing, 1 denotes a transparent insulating substrate made of quartz, glass, or the like, in which glass is used normally in terms of cost; 2 denotes a polycrystalline silicon thin film; 3 denotes a gate insulating film; 4 denotes a gate electrode; 5 denotes an interlayer insulating film; 6 denotes a source electrode film; 7 denotes a drain electrode film; and 13 denotes an underlying layer (so-called undercoat), which is formed with the view to preventing some components of a substrate materiel from being diffused in the polycrystalline silicon thin film but may not be formed in some cases depending on the substrate material or a method of processing the substrate.
In practice, such thin-film transistors used as switches for the pixel portion and in driving circuits therefor are arranged in rows and columns in vertical and lateral directions and at locations determined by the display surface of a liquid-crystal display device as a product and by the driving circuits formed in the peripheral portion thereof. However, since the foregoing is so-called well-known technology and is not relevant directly to the present invention, intentional depiction thereof is omitted.
A method of fabricating the thin-film transistors, which is relevant directly to the present invention, will be described briefly herein below, though it is so-called well-known technology.
First, a silicon dioxide thin film 13 is formed as the underlying layer on the transparent insulating substrate 1 made of glass or the like by plasma chemical vapor deposition (PCVD), sputtering, or the like.
Then, an amorphous silicon thin film is formed entirely over the substrate or at a specified location thereon by PCVD, chemical vapor deposition (CVD), or sputtering.
Next, an excimer laser is applied to an amorphous silicon thin film thus formed to temporarily melt the amorphous silicon thin film (so-called laser annealing), thereby forming the polycrystalline silicon thin film 2 composed of grains (particles) each having a relatively large diameter by utilizing the crystallization of silicon during the solidification thereof.
Next, the polycrystalline silicon thin film is processed into a specified configuration determined by the arrangement of transistors (elements) on the substrate. In short, so-called patterning is performed.
Next, the gate insulating film 3 is formed on the patterned polycrystalline silicon thin film by normal pressure CVD, PCVD, sputtering, or like method and the gate electrode 4 is formed at a specified location on the gate insulating film 3.
Next, the interlayer insulating film 5 is formed and contact holes are formed by etching in the portions of the interlayer insulating film in which the source and drain electrodes of each of the transistors are to be formed.
Next, the source and drain electrodes 6 and 7 of each of the transistors are formed by using the contact holes, whereby the polycrystalline silicon thin-film transistor is produced.
It will be appreciated that, if necessary, the cleaning of the substrate, the implantation of impurity ions required by the element to perform its intrinsic function, i.e., phosphorus (P) or boron (B) ions into the source and drain regions, a heat treatment subsequently performed to join a dangling bond or expel excess hydrogen, wiring required by the element to perform its intrinsic function, and the like are performed in addition to the foregoing process steps. Since these process steps are also well-known technology and not relevant directly to the present invention, the description thereof is omitted here.
A description will be given next to irradiation conditions for laser annealing.
To improve the characteristics of the thin-film semiconductor element, the film should have a large and uniform crystal grain diameter. If the crystal grain diameter is to be increased by laser annealing, it is effective to perform irradiation with high energy or irradiate the same portion several times. As a result of such irradiation, however, the grain diameter loses uniformity, the characteristics of the thin-film semiconductor element vary greatly, or heat is transmitted to the glass substrate to cause the deformation of glass or the diffusion of a glass component into the thin-film semiconductor, so that the performance of the semiconductor element degrades against expectations. It is to be noted that the heat resistance temperature of the glass substrate used in the liquid crystal display device is 600xc2x0 C.
Under the present circumstances, therefore, poly-crystallization is performed by applying a laser under conditions which are a trade-off between the size and uniformity of the crystal grain diameter and the adverse effects of heat on the glass substrate.
In addition to the foregoing, there has been adopted the approach of optimizing the energy density of a laser beam in consideration of the thickness of a silicon film or the like, though the description thereof is omitted here since it is not relevant directly to the present invention.
However, the method encounters the following problems during melting recrystallization.
(1) FIG. 2 is a cross-sectional view of a polycrystalline silicon thin film formed by melting recrystallization involving excimer laser annealing. As shown in the drawing, numerous projections 11 are formed at a surface of the polycrystalline silicon thin film 2, particularly at the grain boundaries. Moreover, tramp materials (xe2x80x9cimpuritiesxe2x80x9d in another technical field) 12, which are unnecessary by nature for the transistor element to perform its intrinsic function, e.g., oxygen in an atmosphere, hydrogen from moisture, boron (B) from glass pieces jumped from a HEPA filter, and the like are taken in by the surface portion.
In this case, these tramp materials are not only located in large quantities in the surface which is chemically and physically unstable during the polycrystallization of the amorphous silicon during which the amorphous silicon is temporarily melted at a high temperature achieved by laser irradiation and then solidified but also segregated at the upper surface of silicon from the lower portion thereof with solidification (aggregate in a large quantity from inside silicon). In particular, the tramp materials are assumed to be segregated in large quantities in the projections, which are chemically unstable because of the segregated tramp materials. If oxygen is a tramp material, it is bonded to silicon as a semiconductor in an extremely complicated and unstable state instead of reacting therewith to form a silicon dioxide. It is to be noted that oxygen forms compounds with silicon, carbon, or the like belonging to Group IV at a ratio of either 1:1 (e.g., a carbon monoxide or silicon monoxide) or 2:1 (e.g., carbon dioxide gas or a silicon dioxide) and therefore does not achieve a constant composition. Under special conditions such as in the surface of the amorphous silicon which is solidified immediately after melted, an extremely complicated compound is formed accordingly.
The use of a material obtained by mixing at most 30% germanium or at most 5% carbon in silicon is also examined at present and the development of the material, which is not pure silicon, is pursued since it has various characteristics of easy crystallization because of its lower melting point and the capability of providing a high mobility. However, since the materials used in these cases are inherently mixtures, the surfaces thereof in particular suffer non-uniformity resulting from projections and depressions and increasing segregation of not only the tramp material but also the intrinsic semiconductor material.
In the top-gate-type transistor, the surface portion is a portion in contact with the gate insulating film. Therefore, the surface projections adversely affect the insulation resistance of the overlying gate insulating film. On the other hand, the segregation of the tramp materials destabilizes the interfacial portion with the gate insulating film for the reason stated above. Both of the surface projections and the segregated tramp materials adversely affect the performance and reliability of the thin-film transistor as an element and may cause variations in the performance or the like of the semiconductor element.
Although ion doping is performed subsequently with respect to the source and channel regions by using PH3 and B2H6 to form a C-MOS structure and a heat treatment is performed for the activation thereof, the presence of the projections impairs the uniformity of the impurity ions implanted. This qualitatively causes variations in the characteristics of the TFT.
(2) Although the trade off irradiation conditions described above ensure the uniformity of the characteristics of the polycrystalline silicon as the active region of the thin-film semiconductor element, the electric characteristics thereof including field-effect mobility are reduced compared with those of a monocrystalline silicon semiconductor element. Accordingly, it is difficult to constantly provide circuit functions sufficient for a future liquid crystal display device.
As a result, it has been demanded to develop a polycrystalline silicon film having no projection at the surface thereof, particularly at the interface with the gate electrode portion, and no segregation of tramp materials after laser annealing or having a surface in a stable state and develop a thin-film semiconductor element with excellent performance.
It has also been demanded to develop thin-film transistors each having excellent electric characteristics including field-effect mobility, while ensuring the uniformity of numerous thin-film semiconductor elements formed on a substrate.
To attain the foregoing objects, during the fabrication of a top-gate-type thin-film transistor (element) in accordance with the first group of the invention, a polycrystalline semiconductor thin film is formed by irradiating an amorphous semiconductor thin film formed on a substrate, particularly a thin film made of silicon or containing silicon as a main component, with a laser beam and thereby polycrystallizing the thin film. The polycrystalline semiconductor thin film is then exposed to an active, reactive gas such that a surface layer thereof is etched away, whereby a surface of the polycrystalline semiconductor thin film is planarized and surface portions in which tramp materials are segregated are removed. Moreover, the type of the reactive gas is determined carefully and the etching process using the reactive gas is performed inventively such that a thin-film semiconductor element with high mobility and high reliability is provided.
During the formation of a top-gate-type transistor on a substrate in accordance with the second group of the invention, an amorphous semiconductor thin film, particularly a thin film made of silicon or containing silicon as a main component, is formed and polycrystallized by laser annealing, similarly to the first group of the invention. However, the second group of the invention is characterized in that means of mechanical polishing or not only mechanical but also chemical polishing is adopted to planarize a surface of the polycrystalline semiconductor thin film and remove surface portions in which tramp material have been segregated.
Moreover, a ceramic-based thin film with a high hardness is formed on the substrate such that the substrate is planarized by mechanical and chemical polishing and the tramp materials are removed. The thin film is also used for proper formation of the semiconductor thin film and for the retention of the strength of the substrate.
For this purpose, the material and thickness of the ceramic-based thin film and the engineering and mechanical properties thereof are determined carefully such that a thin-film semiconductor element with high mobility and high reliability is provided.
The third group of the invention is similar to the foregoing two groups of the invention in that an amorphous semiconductor thin film, particularly a thin film made of silicon or a material containing silicon as a main component, is formed on a substrate and polycrystallized by laser annealing. However, the second group of the invention is characterized in that a heat treatment is performed at a high temperature of 550xc2x0 C. or more and the atmospheric gas during the heat treatment is determined carefully for the planarization of the surface, for the growth of the polycrystalline semiconductor into a larger crystal, and for the removal of the tramp materials segregated in the surface.
The third group of the invention also planarizes the surface portion of the polycrystalline silicon thin film and removes the projections in which the tramp materials have been segregated in the top-gate-type transistor, provides an excellent interface with the gate insulating film, and provides a thin-film semiconductor element with high mobility and high reliability, similarly to the foregoing two groups of the invention. However, the third group of the invention is slightly different therefrom in that it is also applicable to a bottom-gate-type transistor in terms of promoting the growth of a crystal.